ZigBee is a new standard for low cost, low power wireless connectivity based on the IEEE 802.15.4-2003 standard. It is particularly well suited to installed infrastructure deployments, such as building automation, and applications involving with a large number of battery powered sensors interconnected via a backbone of mains power ZigBee routers.
A traditional ZigBee network is able to handle some amount of mobility, such that it can cope if a router is moved or goes down, or if a small number of devices are mobile within the whole environment. When a device moves it acquires a new parent, and sends a broadcast message telling the old parent that it can delete the old record of it.
However in a very large or busy ZigBee network, or in a network made up of multiple sub-networks, the commands to remove records have a strong chance of failing to reach the old parent. Therefore, in a traditional ZigBee network, every time a node gets a new parent there is a strong chance that the record of it stored by the old parent will not get removed properly. Consequently, a lot of old records are left lying around. This uses a lot of memory, which quickly becomes full on a low cost device.